Regular skeletal muscle activity, including endurance exercise, is a key determinant of overall well being, and is crucial for countering the metabolic syndrome of obesity, insulin resistance and hypertension. Lack of muscle activity in bed rest, various disease states and aging contributes to the general decline in health under these conditions. Skeletal muscles are composed of different fiber types, and the expression of muscle fiber type-specific genes is modulated by the pattern and extent of activity imposed on each muscle fiber. The mechanisms underlying muscle fiber maintenance and plasticity by repetitive activity are potential targets for pharmacological interventions for preventing deleterious effects of muscle disuse in bed rest, aging or various diseases. Muscle fiber phenotype (ie, fast glycolytic, fast oxidative/glycolytic or slow oxidative), which depends on the identity and level of protein isoform expression in each fiber, is determined by the cumulative pattern of activity that the fiber has undergone over the previous weeks or months. The mRNA levels for the corresponding proteins are altered on a time scale of days or even hours, and the underlying transcriptional regulators are modulated over hours or minutes. This project will examine the molecular signaling mechanisms for activity dependent transcriptional regulation of slow fiber type gene expression in adult fast skeletal muscle fibers using a novel culture system in which calcium, transcription factors and other transcriptional modulators can be localized and quantitatively imaged by confocal microscopy in individual living, fully differentiated adult mouse flexor digitorum brevis (FDB) skeletal muscle fibers maintained and electrically stimulated and/or pharmacologically or molecular biologically manipulated in culture. Our aims will be: (1) To characterize the molecular determinants for movement of the transcription factor NFAT, an activator of slow fiber type genes, into muscle fiber nuclei and into discrete sub nuclear bodies during muscle activity. (2) To determine the molecular mechanisms for the differential regulation of two class II histone deacetylases, HDACs 4 and 5, which bind to and repress the activity of intra nuclear MEF2, a transcription factor which activates slow fiber type genes and which is de-repressed by HDAC movement out of the nucleus. (3) To characterize the muscle activity dependent and independent control of the transcriptional co activator PGC-1 alpha, a key regulator of genes for oxidative metabolism and for slow muscle fiber type. These studies will have important implications for combating the effects of disuse on muscle fiber gene expression in normal sedentary individuals, in individuals with immobility imposed by various diseases and in decreased mobility with aging. PUBLIC HEALTH RELEVANCE. Skeletal muscle activity, including endurance training, is a key determinant of overall well being, and is crucial for countering the metabolic syndrome of obesity, insulin resistance and hypertension. The mechanisms studied here underlie fiber maintenance and plasticity by repetitive muscle activity, and are thus potential targets for pharmacological interventions and for preventing deleterious effects of muscle disuse in bed rest and/or aging. The results of these studies will thus have important implications regarding the effects of use and disuse on muscle fiber gene expression in sedentary normal individuals, in individuals with immobility imposed by a variety of disease states and in decreased mobility with aging.